In U.S. Pat. No. 4,604,111, a particulate contamination control method and filtration device is disclosed. This patent relates to the serious dangers associated with persons breathing particulate contaminated air. The invention is useful for protecting the outside environment and the individuals working in a highly contaminated area such as occurs when asbestos coatings are removed inside a building structure.
Asbestos fibers fall into the generic classification of hazardous particulate and are a well known carcinogenic hazard to humans and animals. Typical environmental standards refer to fibers that are 5 microns or greater in length with an aspect ratio of 3 to 1 or greater. The average asbestos fiber is about 0.1 micron in diameter. It is now accepted that the thinner fibers are the most dangerous threat to human health. The asbestos fibers, in particular those that are thinner and shorter, remain airborne for considerable lengths of time and contaminate large volumes of air to form a substantial hazard to the environment and to the persons working or living in the area.
U.S. Pat. No. 4,604,111 provides a system and method of containing, lowering, and essentially eliminating the danger of asbestos inhalation by workers in a building in which asbestos fibers are generated at extremely high levels. Asbestos fibers are prevented from being released into the outside environment during an unforseen accident such as a leak through a damaged film barrier and by a flap seal which seals an inlet to a contaminated work area through which workers pass to gain entrance to and exit from the work area.
The flap seal is formed in a plastic wall defining the work area. The "flap seal" includes a two foot by five foot opening about a foot off the floor in a sheet of plastic film, such as polyethylene, plasticized polyvinyl chloride or the like, sealed across an existing door frame of the work area. A polyethylene sheet is sealed across a door frame with an opening cut through the film. The flap is larger in all dimensions than the opening of the same film, is attached to the door frame above the opening and hangs over the full length of the opening such that air and the workers may pass into the work area through the opening, pushing flap inwardly. However, the configuration is such that once the air flow ceases and positive air pressure develops in the enclosure, the flap falls into place and air is prevented from escaping in the opposite direction to the environment outside of the work room.
In the 4,604,111 patent, the flap seal is designed to seal the inlet into the contaminated work area. This arrangement has proved very effective in preventing escape of asbestos fibers in the event of loss of negative pressure. However, other factors need to be considered in designing a combined entranceway and exit from a contaminated work area.